This invention relates to a temperature control circuit for an oven, and more particularly to such a circuit for an oven enclosing a crystal which controls the frequency of an electronic oscillator.
A crystal frequency standard of the highest precision requires that the crystal temperature be stabilized and controlled above the upper ambient temperature that the unit will encounter. Unfortunately, when the oscillator is turned on, a warm-up period (minutes) is required before the unit can be used. For many equipments it is highly desirable to minimize this warm-up period. Consequently, large quantities of heat are initially used to bring the oven up to the operating temperature. As larger and larger amounts of heat are used, it becomes increasingly difficult to control the shut-off point for the heaters and to effect a smooth transition to the steady state heaters. The rate of temperature rise may well be several degrees per second and an error in the shut-off time of even a fraction of a second can cause considerable overshoot or undershoot in the temperature. The problem is compounded by the internal temperature rise in the heaters themselves, and if the quick warm-up power is shut off when the outside of the heaters are at the proper temperature, the inside of the heaters being considerably hotter will cause the temperature to increase further.
The problem in brief, is that when a crystal oven is warmed up very rapidly, it is difficult to turn off the quick warm-up power before the temperature of the material inside the heaters and the oven mass in the immediate vicinity of the heaters overshoots the temperature set point.